Individuals recently HIV infected often have high viral loads and may be highly infectious. Risky sexual behavior soon after infection therefore carries high potential for HIV transmission to others. Upon awareness of recent HIV infection, some but not all individuals change behaviors to reduce their risks of transmitting HIV. Possible mediators of such behavior change include drug use and dynamics of partnerships. Following individuals with recent HIV infection and their partners over time can provide information critical to secondary prevention on how risks taken with partners vary during a time of high infectiousness and allow for estimates of how the spread of HIV is most efficient. The proposed study will extend the behavioral research conducted in the NIAID funded Acute Infection and Early Disease Research Program (AIEDRP) known as The Southern California Primary Infections Program and measure how transmission risks and partnership dynamics change overtime among recently HIV-infected individuals and their partners. We will compare behavioral patterns of recently HIV infected individuals with those with chronic HIV infection and no HIV infection. It will allow for partnership level analyses by actively recruiting sexual partners. Of special focus will be the role of drug use, especially methamphetamine, in affecting behaviors over time and how partnership dynamics interact with drug use to allow for HIV transmission. The proposed study will enroll 150 recently HIV-infected individuals and follow them for one year. We will also recruit at up to 6 sexual partners for each recently HIV infected individual and follow these partners in a "cyber-cohort". Some sexual partners will be HIV negative and some HIV positive allowing for comparisons of individual behaviors over time by HIV status in the cohort and between serodiscordant and seroconcordant partnerships. The results will feed into mathematical models that will quantify population-level and partnership-level impacts of behavior change by recently HIV infected individuals, using microsimulation to integrate our results with current estimates of temporal infectiousness patterns. Understanding these associations will be critical for interventions that target risk reduction for recently HIV-infected individuals in serodiscordant partnerships. Studying partnerships and transmission within them may elucidate how some men who have sex with men (MSM), especially minority MSM, may bridge HIV across networks and ultimately into the general population.